Packaging: Drop systemd BuildRequires in case of building for SUSE

Packaging: Drop systemd BuildRequires in case of building for SUSE

There is already pkgconfig(systemd) present, which, in SUSE's packaging,
translates to the same package (systemd.pc is shipped as part of the main
systemd package). Not explicitly mentioning 'systemd' as package name allows
the openSUSE Build Service though to find shortcuts by using the
bootstrap packages, i.e. systemd-mini (ABI/API complete).

Signed-off-by: Dominique Leuenberger <dimstar@opensuse.org>
(cherry picked from commit b48ea142de50ea7b0242e7f478578b9ee8e31b8f)